Answer:
26 apples
Step-by-step explanation:
bananas, b= $0.50 
oranges, o= $0.75
apples, a = $0.75  
Total fruit sold = 50
Total Earned  = $33.50
b + o + a = 50       Because the total number of all 3 fruits sold is 50
.5b + .75o + .75a =  $33.50           The number of each fruit, multiplied by the price of each fruit, and added together is $33.50
They sold twice as many bananas than oranges.
If o is oranges, then 2o = b because there are twice as many bananas so:
b + o + a = 50 
2o + o + a = 50  
3o + a = 50
Now we can solve.
3o + a = 50
a = 50 - 3o
.5b + .75o + .75a =  $33.50
.5(2o) + .75o + .75a = $33.50
o + .75o + .75a = $33.50
1.75o + .75a = $33.50
Two equations and 2 unknowns.
a = 50 - 3o
1.75o + .75a = $33.50       Plug in a to solve for o.
1.75o + .75(50 - 3o) = $33.50
1.75o + .75*50 - .75*3o = $33.50
1.75o + 37.5 - 2.25o = $33.50   Combine like terms
1.75o - 2.25o +37.5 = $33.50
-.5o + 37.5 = 33.5     Add .5o to each side so it cancels on the left.
37.5 = 33.5 + .5o   Subtract 33.5 from each side
37.5 -33.5 = .5o
4 = .5o   Divide each side by .5
4/.5 = o
8 = o
Now plug into a = 50 - 3o
a = 50 - 3(8)
a = 50 - 24
a = 26
So even though we know they sold 26 apples, let's finish solving to check our work.
b + o + a = 50 
b + 8 + 26 = 50 
b + 34 = 50
b = 50 - 34
b = 16       
b = 2o   
b = 2 (8)
b = 16   Which works! Now the last step of checking our work:
.5b + .75o + .75a =  $33.50
.5(16) + .75(8) + .75(26) =  $33.50
8 + 6 + 19.5 = $33.50
14 + 19.5 = $33.50
33.5 = $33.50  Yes! It works too!
bananas = 16
oranges = 8 
apples = 26